The invention is an atmospheric electron x-ray spectrometer. More specifically, the invention is an apparatus for performing in-situ elemental analysis of surfaces.
Prior methods used to perform in-situ elemental analysis, such as alpha proton x-ray spectroscopy and x-ray excited fluorescence have very long spectrum acquisition times. Acquiring a spectrum from a sample has taken several hours using systems in the prior art. Further, the methods used by prior art systems have been limited to analyzing large surface areas of a sample.
Thus, there is a need for a miniature, portable instrument capable of rapidly conducting in-situ elemental analysis of samples by using a compact, low power, for example battery power, instrument.
There is further a need for such a system for determining the chemical and elemental composition of soil and rocks in remote locations not easily accessible by humans, such as in deep wells bored for oil exploration or on Mars and other planetary bodies. Such an instrument must be of minimal size and mass, and for planetary exploration applications, must have a low energy requirement so that the instrument may be included on planetary landing craft.
The above problems are solved by the present invention, which comprises an apparatus for performing in-situ chemical and elemental analysis of surfaces. The invention comprises an atmospheric electron x-ray spectrometer containing an evacuated electron column which generates, accelerates, and focuses electrons and is isolated from the ambient atmosphere by a thin, electron transparent membrane. After passing through the membrane, the electrons impinge on the sample in atmosphere to generate characteristic x-rays. These x-rays are detected and analyzed using a detection system consisting of a solid-state, positive-intrinsic-negative (PIN) diode coupled with an amplifier and a multi-channel analyzer. The output of the x-ray detection system is an x-ray spectrum, which can be analyzed to determine the elemental composition of the surface.
The characteristic x-rays are excited and analyzed to determine the sample elemental composition as in a conventional electron microscope, without the need to introduce the sample into vacuum. The approximately 1 kg instrument may require less than 50 J per acquired spectrum, with a spatial resolution of less than 1 mm and a spectrum acquisition time of less than 1 minute. This rapid analysis capability coupled with the low energy consumption of the instrument enables several terrestrial in-situ measurement applications requiring portable instruments as well as for short duration experiments on space missions with severe constraints on mass and energy resources, and allows, rapid, multiple composition measurements to be made. The high spatial resolution measurements of the surface elemental composition of individual samples made with the instrument will have terrestrial benefits and may also further space exploration program goals to assess the mineralogical and biological state and evolutionary history of pristine or prepared planetary samples.
The electron column employed by the atmospheric electron x-ray spectrometer is that currently used in television tubes, and it can be miniaturized further by microfabrication techniques based largely on the micromachining of silicon. In this embodiment, the electron column comprises stacked wafers, or chips, assembled in a vacuum using a wafer-to-wafer bonding approach. The chips have metal-film apertures that serve as electrodes for accelerations, deflection, and to focus the electron beam through the electron transparent membrane and onto the sample.
The x-ray detector for the apparatus is mounted outside the vacuum and near the sample to capture the spectral signature of the sample.
The invention can be utilized as a sub-kilogram, in situ instrument enabling rapid, quantitative elemental analysis of planetary surfaces. The instrument falls in the same class of miniature in-situ x-ray fluorescence instruments such as the alpha proton x-ray spectrometer (APXS) and other commercially available x-ray induced fluorescence (XRF) instruments. Unlike those systems, the atmospheric electron x-ray spectrometer offers significantly faster spectrum acquisition, much higher spatial resolution, and shorter sampled depth in comparison to the other elemental analysis techniques. These features enable new types of observations of planetary surfaces that were not possible previously.
One feature is a small irradiated spot size. An instrument providing a 100 xcexcm to several mm spot size on the irradiated sample is able to determine local elemental composition of rocks and soil, which has not yet been achieved on another planet. For comparison, the Pathfinder alpha-proton x-ray spectrometer measured a spot about 4 cm in diameter. The small spot size is particularly effective when used in combination with other instruments, such as high resolution imaging systems boresighted with the electron beam. Elemental composition measurements will provide important clues on the temperature, pressure, and other properties relevant to formation and modification conditions of the rock and soil. For example, the amount of Ca, Mg, and Fe in pyroxene, or Fe, Ti, and other cations in iron oxides can be determined.
Another feature is a rapid spectrum acquisition. Short spectrum acquisition times (resulting in low energy consumption) will enable rapid multiple readings of a sample. Alternatively, the invention can be used to rapidly scan several samples, effectively providing a quick look, elemental surface analysis of a region.
Another feature is a short penetration depth into the sample for the electron beam. The xcexcm-scale penetration depth will allow surface coatings and weathering rinds on rocks to be measured with minimal mixing effects from deeper material. Measurements of unaltered surfaces will require prompt sampling of freshly cored or broken rocks, activities that are planned in several future Mars sampling inissions, which can be accomplished by the invention.
The planned reductions in the size of future spacecraft will potentially result in reductions in payload capacity. Therefore, to maximize the science return it is extremely important to develop a suite of highly capable miniature instruments. The atmospheric electron x-ray spectrometer satisfies this need in the area of elemental analysis. One of the most important in situ measurements is the determination of the composition of planetary bodies and remnant planetary building blocks such as comets and asteroids. These measurements can also provide information on prebiotic chemistry in the solar system. X-ray fluorescence techniques provide a non-contact method of determining the elemental composition. When a material is irradiated by high-energy xcex1-particles, x-rays, or electrons, it emits an x-ray spectrum that consists of characteristic peaks for the individual elements (plus a broad background). By using suitable spectrum analysis techniques, not only can the elemental composition be identified but also the mass fractions of the individual elements can be determined to within a few percent. This technique has proved invaluable for determining the chemical make-up of a planetary body, and all in situ missions, including the US Viking 1and2, Mars Pathfinder, and Soviet Venera missions, have carried some form of x-ray fluorescence instrument.